The present invention relates to a method and apparatus for detecting cracks in honeycomb structures that are used in various types of filters and catalyst carriers and, in particular, to a method and apparatus which are simple and easy and highly efficient in detecting cracks in honeycomb structures, and in addition, hardly damage the honeycomb structures.
Generally, honeycomb structures are such that they are cell walled off by cell walls 2 crossing each other and have a plurality of cells 3 penetrating from their one end face 4 and to the other 5, as shown in FIGS. 4(a) and 4(b), and are often used in filters and catalyst carriers. For example, they are used in exhaust gas purifying units for heat engines such as internal combustion engine or combustion equipment such as boiler, liquid or gaseous fuel refining units, and water and sewage treatment units. They are also used as a diesel particulate filter (hereinafter referred to as DPF) to trap and remove particulate substance in dust-containing fluids such as exhaust gas exhausted from diesel engines.
Honeycomb structures used for such objects function to trap and remove unnecessary particulate substance in fluids to be treated or, where they carry catalysts on their surface, to bring fluids to be treated into contact with the catalysts, when the fluids pass through them.
Generally, honeycomb structures can be formed by bringing a powder of ceramic, metal or the like together with a binder etc. to the clay-like state, giving the clay-like mixture a honeycomb shape, and firing the honeycomb-shaped material. In this production process, defects such as crack can sometimes occur in the honeycomb structures. When cracks occur in the cell walls of the honeycomb structures, they cause deterioration in filtration efficiency and strength of the honeycomb structures and prevent the honeycomb structures from achieving good performance.
The simplest method for inspecting cracks in such honeycomb structures is to inspect them visually. However, it is difficult to visually inspect cracks occurring in the cell walls of honeycomb structures. Particularly with decreasing cell wall thickness and increasing cell density of honeycomb structures in recent years, the visual inspection becomes more and more difficult. In case of a DPF, it is made up of cells 3 which are sealed up alternately, as shown in FIG. 5, and porous cell walls 2 (refer to FIG. 4(b)) which trap and remove particulate substance. Accordingly, cracks occurring in the cell walls 2 (refer to FIG. 4(b)) are difficult to observe visually from outside. In FIG. 5, the cells 3′, solid black portions, indicate the plugged cells on one end face, while the cells 3, white portions, indicate the plugged cells on the other end face.
There has been known another method, which is referred to as soot print method, for inspecting cracks in such honeycomb structures. The method is to detect cracks by flowing soot matter produced by the combustion of a diesel fuel into a honeycomb structure, for example, a DPF through its lower face, while covering the upper face of the same with white cloth, so that the soot matter discharged through the upper face adheres to the white cloth. This method is simple and easy and excels in crack detection level, but on the other hand, since it uses soot matter, it requires a step of removing the soot matter having adhered to the honeycomb structure by heat treatment after the inspection. The after-treatment of removing the soot matter requires extra time. Furthermore, the method does not allow the detection of immanent defects such as cracks not penetrating the cell walls, though it allows the detection of cracks penetrating the same. However, such immanent defects must also be detected since they are apt to grow into cracks which penetrate the cell walls when the honeycomb structure is subjected to thermal shock etc. during its use, and moreover, they contribute to the deterioration in strength of the honeycomb structure. Still further, the method does not allow the detection of cracks 8 which penetrate the plugged cells 3 on the same end face, as shown in FIG. 7.
There has been known still another method in which the presence or absence of cracks is inspected by applying air pressure to one end of a honeycomb structure while keeping the honeycomb structure in water and observing the bubbling phenomenon occurring at the other end of the honeycomb structure. In this method, though the after-treatment takes less time than in the above described soot print method, the honeycomb structure must be dried after inspection and degassed in water before inspection. These drying and degassing steps take time. Further, this method allows the detection of neither immanent defects nor cracks which penetrate the plugged cells on the same end face. The present inventors examined the use of optical and acoustic techniques in detecting cracks in honeycomb structures, but satisfactory results could not be obtained in both cases.